The Millimeter Wave (mm-Wave) communication is an emerging technology endowed with large spectrum resources as the technology operates on one or more frequency bands between 30 GHz and 300 GHz. Radio transmissions under such high frequencies would result in large free-space loss for the transmissions. Since the short wavelengths of mm-Wave signals would result in short spacing between antenna elements, the quantity of antenna element packings in an antenna module may escalate as the result of the increase of operating frequency. Consequently, dense antenna elements may result in antenna arrays having radiation patterns with high directivity and large beamforming antenna gains. According to Friis free-space equation, a directional antenna with a high antenna gain would be able to make up the free-space path loss. Recent studies has also shown that high gain antenna is able to overcome the free-space loss achieving over 100 m communication range, even in non-line-of-sight (NLoS) channels.
However, wireless communications using directional antennas would require transmissions in appropriate directions. Such a constraint has led to the initial discussions to cope with challenges associated with reference signaling designs. Even though wireless PAN and wireless LAN on mm-Wave such as IEEE 802.15.3c and IEEE 802.11ad have been developed, a MAC protocol using directional antenna in mm-Wave may nevertheless require re-designs in order to be incorporated in a cellular system.
As the Millimeter Wave technology would likely be adopted as the communication technology of the next generation, a base station operating under millimeter waves would be required to strategically design directional antennas to concentrate transmission powers in particular directions in order to provide the optimum coverage. As an example, FIG. 1 illustrates a communication system which uses Millimeter Wave technology. In the example of FIG. 1, a base station 101 may serve an individual user equipment (UE) such as a mobile phone 102 or a vehicle 104 or may serve UEs operating within a network such as a device to device (D2D) communication network 103 with millimeter waves. In such scenario, a base station would need to know which direction to transmit in order to cover all the UEs because antennas having directivities are required in order to mitigate severe path losses as previously mentioned. Therefore, the direction or position in the argular domain of each of the UEs relative to the base station would need to be known. Also, a base station would need to know the condition of channels between the base station 101 and UEs 102, 103, 104 in order to allocate resources for the UEs.
To obtain the direction of UEs and conditions of channels, a base station conventionally rely upon transmitting reference signals in exchange for channel condition information received from UEs. FIG. 2 illustrates an example of transmitting a reference signal (RS) from a base station 201 and receiving of the reference signal by a UE 202. In response to receiving reference signal, the UE may perform channel estimation (e.g. channel quality indicator (CQI) measurement) and then transmit a feedback signal (S1) to the base station 201. Overall, the process of FIG. 2 could also be used for collecting information about the radio frequency (RF) beam which is used to serve the UE 202 in addition to the measurement of the channel condition between the base station 201 and the UE 202. Consequently, the UE 202 may perform a mm-Wave cell search based on the reference signal (RS), and the base station 201 may be able to perform beam training or beam track based on the feedback signal (S1) from the UE 202.
The reference signaling mechanism of FIG. 2 would be able to support cell discoveries and channel measurements. However, in general, if a base station uses an omni-directional mm-Wave for signaling, the range of the signaling would be shorter than using a directional mm-Wave for signaling, assuming that the maximum transmission power of a base station is a constant. This would potentially lead to the control channels and the data channels having different transmission ranges. If a UE uses directional reception in mm-Wave for reference signaling, it may require beam alignments between a base station and a UE and thus would lead to large overheads.
FIG. 3 illustrate an example of transmitting directional specific reference signals from a base station for a plurality of user equipment situated in different locations. FIG. 3 assumes a 2-dimensional horizontal plot (X-Y plane) relative to the earth's surface. In a typical mm-Wave communication system, a base station 301 may need to serve multiple UEs 311, 312, 313, 314 located in various locations around the base station 301. In order to serve UEs 311, 312, 313, 314, the base station 301 would need to know which beam would best serve any particular UEs 311, 312, 313, 314. More specifically, the base station 301 would need to know the location of the UEs 311, 312, 313, 314 as well as the channel conditions of each of the UEs 311, 312, 313, 314. This could be accomplished by the reference signal mechanism of FIG. 2. A base station 301 may either transmit reference signals through directional beams of different angles or perform an omni-directional transmission for reference signals.
FIG. 4 illustrates an example of transmitting a single directional reference signal from a base station in comparison to transmitting multiple directional reference signals. Assuming that the maximum overall transmission power used by a base station is constant, omni-directional transmission would have a shorter range; whereas a directional RF beam, though having a longer transmission range, would only cover a specific direction instead of all directions. As shown in FIG. 4, having multiple simultaneous directional beams would incur reduced power for each simultaneous beam relative to a single directional beam assuming that power is equally shared among each simultaneous beam. Thus in FIG. 4, there is more power and greater range in the single RF beam used by the base station 401 to scan UE 411 than the each of the four beams sued to scan four UEs 411 412 413 414 simultaneously.
Because of the nature of mm-Wave based 5G systems as described above, transmissions of reference signals may need to be designed according to the mm-Wave characteristics which may require directional beams. Thus, the disclosure proposes a method and a system of transmitting reference signals by taking account of the mm-Wave characteristics.